Layers of lipids in stratum corneum of the skin form a "water barrier" which prevents water loss from the skin. Known classes of stratum corneum lipids include sphingolipids, free fatty acids, sterols and sterol esters, and phospholipids, with sphingolipids being a major component. Sphingolipids, in turn, consist of four major classes of lipids: glycosphingolipids, ceramides, sphingomyelin and total sphingoid base. Ceramides and other sphingolipids play a major role in promoting cell differentiation and, thus, preventing, reducing, or eliminating skin dryness and wrinkles. Although several species of natural ceramides have been identified, these ceramides must be obtained through a lengthy process involving the extraction of ceramides from natural sources. Thus, the availability of natural ceramides is limited and their cost is very high. Several analogs of natural ceramides, known as pseudoceramides, have been synthesized. Pseudoceramides look similar but are not identical to ceramides. Unfortunately, pseudoceramides are still expensive, albeit not as expensive as natural ceramides.
Recent work has shown the ability of cultured keratinocytes to synthesize sphingolipids. Due to the high cost of natural or synthetic sphingolipids, cosmetic compositions which can enhance biosynthesis of sphingolipids are desirable, in order to minimize or eliminate the need for exogenous application of sphingolipids.
Skin treatment compositions are known which contain alpha hydroxy acids as ingredients for improving the appearance of dry, flaky, wrinkled, aged, photodamaged skin and for treating various disorders of skin, e.g., hyperkeratosis, ichthyosis, skin blemishes, acne, warts, herpes, psoriasis, eczema, pruritis.
PCT application WO 93/04669 discloses compositions containing N-acetyl-L-cysteine or derivatives thereof. The compositions are said to efface and prevent wrinkles in mammalian skin.
PCT Application WO 93/10755 discloses compositions containing salicylic acid as an essential ingredient in combination with an additional active component, which may be a N-acetyl-L-cysteine derivative. Many optional ingredients are disclosed, among which alpha hydroxy acids or derivatives thereof are mentioned. The compositions are said to regulate wrinkles and/or atrophy in mammalian skin.
The art discussed above does not address the need for increasing sphingolipid biosynthesis in skin and does not envision skin treatment compositions based on a specific lipid biosynthesis enhancing system of N-acetyl-L-cysteine and alpha hydroxy acid according to the present invention. The art does not teach or suggest that N-acetyl-L-cysteine is capable of enhancing lipid biosynthesis or that alpha hydroxy acids further compliment the action of N-acetyl-L-cysteine. It was found, as part of the present invention, that compositions according to the PCT application 93/10755 do not attain a statistically significant increase in lipid biosynthesis.
Accordingly, it is an object of the present invention to provide skin treatment compositions for increasing sphingolipid biosynthesis in mammalian skin.
It is another object of the invention to provide a composition for improving skin condition by improving water barrier performance.
It is another object of the present invention to provide compositions containing N-acetyl-L-cysteine and an alpha hydroxy acid (or a derivative of an alpha hydroxy acid) as an active system capable of increasing sphingolipid biosynthesis in mammalian skin, while tailoring the rest of the composition so as to minimize or substantially eliminate any blocking effect on the active system.
It is another object of the invention to provide a method for treating or preventing the appearance of wrinkled, flaky, aged, photodamaged skin or skin disorders.
It is yet another object of the invention to provide a method for enhancing lipid biosynthesis in mammalian skin.
It is another object of the invention to provide a method for improving skin condition by improving the performance of water barrier.
These and other objects of the invention will become more apparent from the detailed description and examples that follow.